High-power microwave tubes provide electromagnetic energy from the kinetic energy of the electrons emitted by a diode (electron gun) of the tube. The electrons from the beam are finally collected at one end of the tube by an electron collector.
FIG. 1 shows the trajectories Tj of the electrons of a beam 2 of electrons in the collector 3 of a klystron. The trajectories Tj of the electrons in the collector of the klystron depend on their energy level. The more energy (or speed) the electron has, the higher is its trajectory in the collector. The slowest electrons have trajectories Tj which are more deflected from the base 4 of the collector and are consequently intercepted lower in the collector. The spreading of the beam on the inside face 5 of the collector therefore depends on the energy spectrum of the electron beam. In the case of a gyrotron, the trajectories of the electrons in the collector are principally connected to the magnetic field lines.
The gyrotron is a microwave generating tube, the structure of which is illustrated in FIG. 2. The gyrotron principally comprises, along a longitudinal axis ZZ′ of the tube, an electron gun 12, a magnetic compression section 14, a resonant cavity 16, an injector 18 with a microwave power exit 20 and an electron collector 22.
The electron gun 12 comprises a cathode 26 generating an electron beam 28, along the axis ZZ′ of the tube, which has a crown-shaped section. The beam takes the shape of a hollow tube along the axis ZZ′.
A solenoid 30, at the magnetic compression section 14, generates a magnetic field B keeping the electrons emitted by the cathode 12 in the axis ZZ′ of the microwave tube.
The electron beam, in yielding part of its kinetic energy in the resonant cavity 16, provides an electromagnetic microwave, channeled by focusing reflectors 34 at the exit of the injector 18, to the power microwave exit 20 of the gyrotron. Approximately 20% to 50% of the kinetic energy of the electrons is converted into electromagnetic energy. On exiting the injector 18, the electrons from the beam will strike the walls of the collector 22, heating it through the conversion of their remaining kinetic energy into heat energy.
In the case of the gyrotron, the electron beam is channeled by the field lines Fi of the magnetic field B generated by the solenoid 30. The trajectories Tj of the electrons are trapped by the magnetic field lines and cannot spread naturally in the collector. The field lines Fi, which are substantially parallel in the compression zone, gradually spread further apart from the axis ZZ′ of the tube. The electrons from the beam substantially follow these field lines created by the solenoid 30, and, as a result, the electrons from the beam always impact on a same zone Z1, having a ring shape, on cylindrical inner walls 34 of the collector around the axis ZZ′ of the tube.
For an electron beam carrying, for example, a power of 2 MW, the power density dissipated in this ring sector Z1 of the collector is considerable and requires vigorous cooling of the wall. This cooling can be obtained generally by circulation of water using a large and cumbersome cooling installation.
To limit the power density on the collector, magnetic devices are used in the microwave tubes of the prior art to spread the electron impact zone in the collector.
In a first magnetic spreading device of the prior art, the collector of the tube comprises a collector solenoid 40 generating a magnetic field which is weakly divergent in the direction of movement of the electrons. The effect of this diverging magnetic field is to lay down the trajectories of the electrons to make them almost parallel to the walls of the collector. The impact zone is thus considerably lengthened and therefore has its surface increased which reduces the power density on the surface of the collector. The effect of spatial spreading obtained by the magnetic device can be combined with a periodic sweeping effect of the magnetic field in the collector. To this end, the collector solenoid 40 is fed with a periodic signal Ubl of constant amplitude.
FIG. 3 shows a view of the collector 22 of the gyrotron of FIG. 2 of the prior art comprising a magnetic device 39 for spreading the electrons in the collector.
The magnetic spreading device 39 mainly comprises the collector solenoid 40 which is fed with the sinusoidal sweep signal Ubl of constant amplitude at a frequency Fb. The solenoid 40 then produces, in the collector, a sweep field Bbl which can vary in synchronism with the signal Ubl.
The variation of the field Bbl in the collector causes the electron impact zone to move in synchronism with the same signal Ubl, this impact zone moving between a low impact surface Lz with mean position Lp and a high impact surface Hz with mean position Hp. The mean position Mp can be defined, for example, as the position of a circle located at equal distance from the edges of the beam on the impact surface.
FIG. 4 shows the mean position Mp of the impact zone of the electrons on the collector as a function of time t, passing, at the rhythm of the sweep signal Ubl, from the low position Lp to the high position Hp and then in the opposite direction.
The sweep for spreading the beam produced by the signal Ubl reduces the maximum power density on the inner of the collector as a result of the variation of the position of the beam over time. This type of sweep nevertheless has disadvantages. Indeed, the temperature of the collector surface against which the electrons impact is far from being uniform. FIG. 5 shows the variation in the temperature T of the collector as a function of the mean position Mp of impact of the beam.
This figure shows a large variation in the temperature of the collector depending on the mean position Mp of the beam. The temperature is much higher for the low and high positions Lp and Hp of the electron beam corresponding substantially to the cusp points of the variation of the sweep field, i.e. substantially for the maximum and minimum of the sweep spread signal Ubl.
For example, the temperature of the collector (see FIG. 5) for a power gyrotron can vary between a minimum temperature Tmin of 140° C. between the two cusp points and maximum temperatures Tmax of around 300° C. at the cusp points.